通用Scala函数中类型推断失败

Question

consider a simple function that operates on collections distinctBy , which, like distinct remove "duplicates" (which are not necessary actual duplicates):

import scala.collection.TraversableLike
import scala.collection.generic.CanBuildFrom
import scala.collection.mutable.{Set=>MSet}

def distinctBy[T,R,Coll]
  (xs: Coll)
  (f: T => R)
  (implicit ev: Coll <:< TraversableLike[T,Coll], 
   cbf: CanBuildFrom[Coll,T,Coll]): Coll = {

  val builder = cbf(xs)
  builder.sizeHint(xs.size)
  val seen = MSet.empty[R]

  xs.foreach { elem =>
    if(!seen(f(elem))){
      builder += elem
    }
  }
  builder.result()
}

now consider a class to use it on:

case class X(i: Int, j: Int)

using this function naively fails:

scala> distinctBy(Vector(X(1,2),X(3,2),X(1,1),X(2,2)))(_.i)
<console>:14: error: missing parameter type for expanded function ((x$1) => x$1.i)
              distinctBy(Vector(X(1,2),X(3,2),X(1,1),X(2,2)))(_.i)
                                                          ^
<console>:14: error: Cannot construct a collection of type scala.collection.immutable.Vector[X] with elements of type Any based on a collection of type scala.collection.immutable.Vector[X].
              distinctBy(Vector(X(1,2),X(3,2),X(1,1),X(2,2)))(_.i)
                                                             ^

but if I help the type inferencer, this works:

scala> distinctBy(Vector(X(1,2),X(3,2),X(1,1),X(2,2)))((x:X) => x.i)
res1: scala.collection.immutable.Vector[X] = Vector(X(1,2), X(3,2), X(1,1), X(2,2))

scala> distinctBy[X,Int,Vector[X]](Vector(X(1,2),X(3,2),X(1,1),X(2,2)))(_.i)
res2: scala.collection.immutable.Vector[X] = Vector(X(1,2), X(3,2), X(1,1), X(2,2))

to my best understanding, since the function is given in a second argument list, the type inferencer should have picked up that it's a function from X to something. and since X has a member i of type Int , all should have been OK with the first try. so, what am I missing here?

Answer 1

This simplified version works fine for me:

object A {
  def f1[T, R](l: List[T])(f: T=>R) = None

  case class X(i: Int, j: Int)

  f1(List(X(1,1),X(2,1)))(_.i)
}

As you can see collection in first parameter list has T type that allows scala inference type in second arguments list.

So you need build somehow dependencies between Col and T in your example. Not sure if third implicits parameters list helps here.

UPD. Looks weird but seems it works:

object A {
  def f1[T, R, Col[Z]](l: Col[T])(f: T => R) = None

  case class X(i: Int, j: Int)

  f1(List(X(1,1),X(2,1)))(_.i)
}

UPD2. Rewritten sample from question.

  import scala.collection.TraversableLike
  import scala.collection.generic.CanBuildFrom
  import scala.collection.mutable.{Set=>MSet}

  def distinctBy[T,R,Coll[Z]]
  (xs: Coll[T])
  (f: T => R)
  (implicit ev: Coll[T] <:< TraversableLike[T,Coll[T]],
   cbf: CanBuildFrom[Coll[T],T,Coll[T]]): Coll[T] = {

    val builder = cbf(xs)
    builder.sizeHint(xs.size)
    val seen = MSet.empty[R]

    xs.foreach { elem =>
      if(!seen(f(elem))){
        builder += elem
        seen.add(f(elem))
      }
    }
    builder.result()
  }

  case class X(i: Int, j: Int)

  distinctBy(Vector(X(1,2),X(1,2),X(3,2),X(1,1),X(2,2)))(_.i)
  distinctBy(Map("1" -> X(1,2), "2" -> X(1,2), "3" -> X(3,2)))(_._2.i)